Heliostats are typically used to orient mirrors in systems to convert solar energy into useful energy, for example, electrical power, by reflecting sunlight onto a fixed target. Their orientation is adjusted by means of a mechanical system allowing the rotation of the mirror generally around two axes of rotation. The rotation is typically activated by motors and gears. These motors are controlled by solar trackers that calculate the required position of the heliostat in order to provide adjustment information needed by the system to assure constant reflection of the sunlight onto the fixed target while the relative position of the sun changes during the course of a day.
Whenever a number of such heliostats are used to reflect the sunlight onto the same target, then a considerable amount of energy is concentrated onto a relative small area of the target, and this energy can be efficiently used to provide usable energy. For example, a steam engine coupled to a generator or an array of photocells can be used to produce electrical energy. The high concentration of energy on the target allows the higher efficiency of energy conversion devices, thus reducing the cost of the system. However, this cost reduction is compensated for by the cost of the heliostats. Thus it is advantageous to reduce the cost of heliostats.
The cost of the motors, gears and motion controllers constitutes a significant part of the total heliostat cost. Motors should be controllable, like adding steppers, but this requires an associated costly controller. Gears should be without backlash and linear, i.e., should provide a linear relation between the rotation of a particular motor driveshaft and the amount and angle of rotation of an associated heliostat around a corresponding rotation axis.
Many prior art heliostat systems using stepper motors need adjustments during set-up, since their rotation angle is commanded incrementally, and not by absolute position. This means that each heliostat must be provided with an individual reference point mechanism. This requirement generally entails additional expense, especially, additional calibration costs at installation.
Furthermore, this calibration often needs to be re-done periodically in order to avoid an eventual accumulation of small errors. Such small errors can be due to command noise, imprecision of gears, slight movement of the heliostat base, or some rare failure of the stepper motor to execute a commanded increment. Such a periodic re-calibration also adds operating costs to a solar plant.
Thus there is a need for a solar tracker device and system that is accurate, simple to operate, needs no adjustment, and that is cost-effective.